The present disclosure relates generally to a system for, and a method of, controlling target illumination for an imaging reader that reads targets by image capture, and, more particularly, to controlling the illumination of a target during operation of a solid-state imaging sensor having a rolling shutter that sequentially exposes an array of pixels to capture an image from the illuminated target.
Solid-state imaging systems or imaging readers have been used, in both handheld and/or hands-free modes of operation, to electro-optically read targets, such as one- and two-dimensional bar code symbols, and/or non-symbols, such as documents, over a range of working distances relative to a light-transmissive window provided on each reader. The reader includes an imaging assembly having a solid-state imager or imaging sensor with an array of photocells or pixels, which correspond to image elements or pixels in an imaging field of view of the sensor, and an imaging lens assembly for capturing return light scattered and/or reflected from the target being imaged, and for projecting the return light onto the sensor to initiate capture of an image of each target. Such a sensor may include a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, with global or rolling exposure shutters, and associated circuits for producing and processing electrical signals corresponding to a one- or two-dimensional array of pixel data over the imaging field of view. In order to increase the amount of the return light captured by the sensor, for example, in dimly lit environments or for far-out targets located relatively far from the window, the reader includes an illuminating light assembly for illuminating the target with illumination light over an illumination field for reflection and scattering from the target. The return light captured by the sensor includes the returning illumination light and any ambient light in the vicinity of the reader.
To achieve a desired image brightness, also known as a white level, for the captured image, it is known to use an automatic exposure controller (AEC) to control the sensor's exposure time, and to use an automatic gain controller (AGC) to control the sensor's gain. The AEC and the AGC are, in turn, controlled by a main controller or programmed microprocessor. Increasing the exposure time and/or the gain will increase the captured image brightness. A typical known strategy is to use exposure priority, in which the exposure time is increased first until a maximum exposure time or threshold (typically around 4-8 ms in order to reduce hand jitter motion effects for a handheld reader) is reached. If the image brightness is still too low as determined by the main controller, then the gain is increased. This strategy maximizes the signal-to-noise ratio (SNR) of the sensor, because the gain is only increased when necessary.
The amount of the illumination light delivered to, and returned from, the target by the illuminating light assembly is another factor that contributes to the captured image brightness. The greater the intensity or output power of the illumination light, the brighter is the captured image. It is known to maintain the illumination power supplied by the illuminating light assembly at a maximum or peak constant output power level during the AEC/AGC process.
When using a global shutter sensor where all the pixels are exposed at the same time, it is known to turn the illuminating light assembly on to illuminate the target only during the exposure time. This results in a very efficient use of the illumination light since the illuminating light assembly is turned off when not needed during non-exposure times. As the exposure time decreases, the less illumination light power is used. However, when using a lower cost, rolling shutter sensor where the pixels are sequentially exposed at different times, it is known to turn the illuminating light assembly on throughout the time of an entire frame, regardless of the exposure time, in order to illuminate and capture the entire target image. A typical exposure time is much shorter than the frame time (e.g., for a sensor operating at 30 frames per second, the maximum exposure time could be about 4 ms, while the frame time is 1/30 sec=33.3 ms). This results in a very inefficient use of the illuminating light assembly, especially for sensors having short exposure times and long frames. The additional electrical energy consumed during generation of the illumination light is not only wasteful and energy-inefficient, but also generates undesirable heat, reduces hand motion tolerance, and undesirably drains an on-board battery typically provided in handheld, wireless imaging readers, thereby requiring more frequent recharging, more downtime, and shorter working lifetimes.
Accordingly, there is a need to more efficiently control target illumination in real-time to reduce average illumination power over a frame, conserve electrical energy, reduce generated excess waste heat, and increase hand motion tolerance, in the operation of imaging readers having rolling shutter sensors, which are preferred over global shutters, primarily for cost savings, with a minimal impact on reading performance.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and locations of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.